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lake source heat pump can be very efficient. intallation should be done by a contractor that specializes in this type of unit or one with lots of experience with water source hp. depth of line in pond depends on the climate.
i believe poly tubing in rolled coils is used as the heat exchanger. be careful who you choose to do this because the system component must be sized properly and installed correctly and water will need chemicals

Lake water heat pumps

There are a lot of questions before you can get answers:

Are you planning on using an "open loop" system using lake water directly through the water source heat pump? Or are you planning a closed loop system with a pile of HDPE coils anchored to the lake bottom? Other issues to consider: How steep is the lake bottom where you plan to do this? Are there any navigable waters acts you need to be aware of (adding marker bouys, "no anchorage" signs, etc.) Where is the thermocline? What is the seasonal lake water temperature variation at the depth you plan to either take the water from, or drop in the closed loop coils? What are the local environmental laws and fish habitat protection issues? What is the energy load you plan on using/rejecting to the lake? Gotta watch the temperature rise in summer as you reject heat into the lake, and the temperature drop as you use heat from the lake water in wintertime.

Not knowing much else, I'd strongly suggest a closed loop system with coils of HDPE anchored as deep as possible to get the best steady state lake water temperature exchange, and design the system for the smallest delta T you can in the lake side- ie: size the lake water loops for a maximum of 7F temperature rise or drop so you don't have a detrimental effect on the ecosystem. I'd recommend avoiding a direct open loop system where you take lake water directly into a heat pump or even a heat exchanger due to the maintenance issues (cleaning organic growth, water intake maintenance, etc.)

This type of system isn't that common, so an experienced geo-exchange system designer who can also do the "whole building system" approach to the installation is highly recommended. The first thing to do is to review and check all of the local and state environmental protection issues with regard to using the lake water as a heating and cooling medium, plus the issues of a bunch of HDPE coils anchored to the lake bottom- who owns the lake bottom rights?

No restrictions other than getting it past the board. Wonder if I could convince than that it was just a fancy cropy bed <G>.

Currently have 2 gas furnaces (one is well over sized and the other humgeously oversized) and 2.5 tons AC on the 2nd floor.

So I will be replacing both. If I go that way I am guess 3.5 or 4 tons total.

But the reason for listing the area was to get some idea of how deep I need to get the coils.

Also how big they might be.

At the end of my dock I have just over 4ft of water. Now I can get to 8-10 ft, but that would be a harder sell to the board.

I have a temp probe in the water, about 18" down and the lowest it has gotten is 36.

I have often thought about an open loop water system. But in addtion to your concerns I am also worried about freezing.

At the shoreline the waster is only about 12" the bottom is solid rock.

Interestingly I asked about ground source heat pumps when I built in 79. They where "out there", but 90% of the local people had not heard of them. The other 10% had heard of them, but did not know anything about them.

Lakewater geothermal system

OK, sounds like most, if not all of the environmental and legal crap is a non-issue.

Next: OK so the lowest temperature you measured was 36F, but what about the highest temperatures in the summer? You will be trying to reject heat into the water in summer, so the idea is to try to locate the geo-exchange pipes as deep as possible to get the coolest surrounding water in summer as well. Conversely, in the wintertime, when you want to suck heat out of the lake, you want the geo-exchange pipes in non-freezing water that will be as warm as possible. Normally in winter the top surface depths of the water in a lake get colder than the deeper water. The thermocline level in the lake is what determines the depth at which the deeper water usually stays at around 39F year round. The depth of the thermocline varies with season and wind/wave action. In an ideal situation, the geo-exchange loops want to be below the thermocline so you can design the system for a more or less constant water temperature condition.

Shallow geo-exchange loops can be used, but they have to be sized for the worse case - the local water temperature in summer when you are trying to reject heat to the water, or winter, when you want to take heat from the water. If the lake is generally fairly shallow and there isn't enough depth near you to make use of the thermocline, you are in risky territory due to the lake water temperature fluctuations you will have to design for. This may double the size of the geo-exchange coils. The "general" (and I mean "general"!) rule of thumb is that you'd need about 200 feet of geo-exchange tube length per ton of cooling (or per 12,000 Btuh of heating). Depends which is the biggest load - do you need more cooling than heating? Or is the heating load what the heat pumps will be sized for?

The "200 feet" rule applies for just plain length, even if the tube was all coiled up. The actual tube length, size and configuration is location specific and installation-specific, so your local condition may dictate more , or less tube length per ton, depending on the load profile, energy balance, and water temperature conditions from season to season.

Closed loops are better..

Pond loops can be a real problem if water level drops or algee develops.An open loop should be min. of 15' if coiled and 10' lateral line loop.
I would suggest that you find a Geo-Thermal contractor in your area,and do research of climate of your area.

If you have an open loop and water falls and the loops are exposed, all of savings are gone.You may want to think about drilling 4 holes vertical,then bring them into the residents piped to (1-2)systems.

"Everyday above ground, is a good day".
"But everyday that you have made a difference in someones life, may insure you stay above ground a little longer".<aircooled>

it was tried here with a condominium complex, worked great for a few years, then the problems started. within 8 years all systems had been replaced with regular heat pumps. nothing like telling a customer, sir your pipes leaked and your unit is full of slime, sir you see that pipe floating out there, well thats your problem, or sir algee doesn't have a very good heat transfer rate. but the best one was, sir, i don't know how long it will take the pond to refill, why don't you ask the vandals who opened the dam how long it took to empty, and yes i know there is a drought going on.

I will have to talk to some of the fisherman to see if they know what how the thermoclime runs.

Upper water gets about 80.

I am beginning to see that unless I can get deep enough to where I can get 50-60 year around temps I won't be much do a whole to better than air to air and then I have the overhead of the water circulation system. Specially installation cost.

You have given me some good base information. There are some homeshows the next few weeks and I am going to see if any of the dealer there have done this.

"do you need more cooling than heating? Or is the heating load what the heat pumps will be sized for?"

We are an EMSW Climate (equally misserable summer and winter). We have 3 nice days in the spring and 4 in the fall <G>.

My guesstimates, based on the runtime and nameplate specs makes me thing that the upstairs needs about 36k btu for heat and the current 2.5 ton AC is perfect (if it did not leak).

How much "tweaking" can you do to a system to have more heat than AC (other than a 2stage system). I know that size of the AC size is very important to maintain good humidity control. And in KC that is critical. I typically don't turn the AC on until it is well over 90 UNTIL THE HUMIDITY LEVEL GOES UP. Then it stays on until it breaks in the fall.

Of course this all needs to be worked out with calcuation before signing on the dotted line.

The downstairs furnace is so oversized that I can't even make wild guess on how much heat that I need and it currently does not have AC. And I will still typically run it that way. But I can bring it on for those 5-10 days that are really bad.